Above the critical temperature, the vapour and liquid of a substance have the same density and the fluid cannot be liquefied by increasing the pressure; the supercritical fluid has density and solvating power approaching that of a liquid, viscosity similar to that of a gas, and diffusivity intermediate between those of a gas and a liquid. A typical phase diagram is shown in Figure 1. It follows that supercritical fluids have

Critical point Supercritical

Gas I

Triple point


Figure 1 Typical phase diagram of a single substance. (Reproduced with permission from Clifford T(1998) Fundamentals of Supercritical Fluids. Oxford: OUP.)

properties which make their use as chromatographic mobile phases very favourable. Thus, gas chromatog-raphy (GC) produces narrow peaks and high resolution, but cannot be used for involatile or thermally labile compounds since analytes must be in the vapour phase. High performance liquid chromatogra-phy (HPLC) is extensively applied to involatile compounds, but has limited resolution because the high viscosity of the mobile phase limits column length, and the low diffusivity slows the exchange of analyte molecules between liquid streams of different velocity. Supercritical fluid chromatography (SFC) addresses these limitations because of increased solubility (which varies with density and hence applied pressure) for involatile compounds in comparison with GC, and lower viscosity and higher diffusivity in comparison with HPLC.

Both capillary and packed columns are used in SFC, and many of the detectors of both GC and HPLC may be employed. Coupling to spectroscopic detectors has proved fruitful. SFC has established a number of niche areas: it extends the range of GC, it can replace many normal phase HPLC methods and it has many advantages over size-exclusion chromato-graphy.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook

Post a comment